A liquid crystal display apparatus performs displaying by varying the optical anisotropy of a liquid crystal layer, and thus varying the transmittance of light, in accordance with a voltage which is applied across the liquid crystal layer. Depending on the light which enters the liquid crystal layer when displaying, liquid crystal display apparatuses are generally classified into three types. The three types of liquid crystal display apparatuses are: transmission type liquid crystal display apparatuses, reflection type liquid crystal display apparatuses, and transflective type liquid crystal display apparatuses.
In a transmission type liquid crystal display apparatus, a backlight is disposed on the back face of a liquid crystal display device, and light from the backlight is transmitted through the liquid crystal display device so as to be perceived by the user. In a reflection type liquid crystal display apparatus, incident light through the front face is reflected by a liquid crystal display device so as to be perceived by the user. A transflective type liquid crystal display apparatus functions in a similar manner to either a transmission type liquid crystal display apparatus or a reflection type liquid crystal display apparatus, depending on the environment of use. Specifically, in an environment where externally-entering light is strong, a transflective type liquid crystal display apparatus functions similarly to a reflection type liquid crystal display apparatus; and in an environment where externally-entering light is weak, the backlight is activated, and the transflective type liquid crystal display apparatus functions similarly to a transmission type liquid crystal display apparatus. Alternatively, irrespective of the intensity of externally-entering light, a transflective type liquid crystal display apparatus performs displaying simultaneously in the two modes, i.e., a transmissive mode and a reflective mode.
In the following description, a transflective type liquid crystal display apparatus will be exemplified as a liquid crystal display apparatus.
In a commonly-used transflective type liquid crystal display apparatus, the efficiency of utilization of light from the backlight is not so high because reflection electrodes are provided. Therefore, in order to obtain the same luminance as that of a transmission type liquid crystal display apparatus which is similar in construction to the transflective type liquid crystal display apparatus, it is necessary to increase the light intensity of the backlight, thus resulting in a large power consumption. Moreover, if the regions in which transmission apertures for transmitting the light from the backlight are formed are increased, the efficiency of utilization of light from the backlight can be enhanced, but in this case the regions in which the reflection electrodes are formed are decreased, thus resulting in a low efficiency of utilization of external light.
Therefore, in a conventional liquid crystal display apparatus, a microlens array is provided between the liquid crystal display device and the backlight unit (Patent Document 1). In this liquid crystal display apparatus, a microlens array converges light at transmission apertures of a liquid crystal display device, whereby the efficiency of utilization of light is enhanced and thus an appropriate luminance can be obtained without increasing power consumption.
In another conventional liquid crystal display apparatus, a prism sheet having a sawteeth-shaped prism surface is provided on a backlight unit (Patent Document 2).
FIG. 16(a) is a schematic cross-sectional view of a backlight unit 310 which is disclosed in Patent Document 2.
The backlight unit 310 includes: a surface light source 301 for emitting light; a prism sheet 304 for reflecting light from the surface light source 301 in the surface normal direction; and a reflector 305 which is disposed on the opposite side of the surface light source 301 from the prism sheet 304.
The surface light source 301 includes: a light source 302 for emitting light; a reflection member 302a provided so as to surround the light source 302; and a light guiding member 303 for receiving the light emitted from the light source 302 and propagating the received light. The reflection member 302a reflects the light emitted from the light source 302 toward the light guiding member 303, and allows the light from the light source 302 to efficiently enter the light guiding member 303.
The light guiding member 303 includes: a principal face (front face) 303a opposing the prism sheet 304; a principal face (rear face) 303b opposing the reflector 305; and an end face 303c which connects the front face 303a to the rear face 303b. The end face 303c of the light guiding member 303 receives the light emitted from the light source 302, such that the light propagates in a propagation direction P while being repetitively reflected between the front face 303a and the rear face 303b of the light guiding member 303. The front face 303a of the light guiding member 303 is parallel to the propagation direction P. As shown in FIG. 17, the rear face 303b of the light guiding member 303 includes regions which are tilted by a tilt angle α with respect to the propagation direction P.
FIG. 16(a) is referred to again. The prism sheet 304 is disposed at the front face 303a side of the light guiding member 303. The prism sheet 304 has a sawteeth-shaped prism surface, the prism surface having alternately-disposed prism slopes 304a and second prism slopes 304b. The prism surface of the prism sheet 304 opposes the front face 303a of the light guiding member 303. Ridges 304c are formed at boundaries between the first prism slopes 304a and the second prism slopes 304b. In the prism sheet 304, as shown in FIG. 18, each vertex angle between a first prism slope 304a and a second prism slope 304b is θ.
FIG. 16(a) is referred to again. In the backlight unit 310, light emitted from the light source 302 enters the light guiding member 303, and propagates inside the light guiding member 303 in the propagation direction P.
As shown by the broken line of FIG. 19, when light enters an interface between the front face 303a of the light guiding member 303 and the air at an incident angle which is smaller than a predetermined incident angle (angle of total reflection), a portion of the light is refracted at the interface between the front face 303a and the air so as to be emitted forward, and a portion of the light is reflected at the interface between the front face 303a and the air.
On the other hand, as shown by a solid line in FIG. 19, when light enters the interface between the front face 303a of the light guiding member 303 and the air at an incident angle which is equal to or greater than the angle of total reflection, the light is totally reflected.
FIG. 16(a) is referred to again. The light which has been refracted at the interface between the front face 303a of the light guiding member 303 and the air enters the prism sheet 304, and is reflected in the surface normal direction by the prism sheet 304.
Moreover, the light which has been reflected at the interface between the front face 303a of the light guiding member 303 and the air travels toward the rear face 303b of the light guiding member 303, and the light which is refracted at the interface between the rear face 303b of the light guiding member 303 and the air is emitted rearward, so as to strike the reflector 305. This light is reflected by the reflector 305, and again enters the rear face 303b of the light guiding member. Thereafter, the light is emitted forward from the front face 303a, enters the prism sheet 304, and is reflected by the prism sheet 304 in the surface normal direction.
Thus, in the backlight unit 310, the light which has once been emitted from the rear face 303b of the light guiding member 303 is reflected by the reflector 305 so as to be used for displaying, whereby the efficiency of utilization of light from the light source 302 can be enhanced. Moreover, since the prism sheet 304 is provided in the backlight unit 310, highly-directional light can be emitted for a liquid crystal display device (not shown).    [Patent Document 1] Japanese Laid-Open Patent Publication No. 11-109417    [Patent Document 2] Japanese Laid-Open Patent Publication No. 11-224058